Embryology is a branch of comparative anatomy which studies the development of vertebrate animals (animals with backbones) before birth or hatching. Embryos are offspring in the process of development, before all vital structures have developed. Like adults, embryos show similarities which can support common ancestry. For example, all vertebrate embryos have gill slits and tails, as shown in pictures below. The “gill slits” are not gills, however. They connect the throat to the outside early in development but eventually close in many species; only in fish and larval amphibians do they contribute to the development of gills. In mammals, the tissue between the first gill slits forms part of the lower jaw and the bones of the inner ear. The embryonic tail does not develop into a tail in all species; in humans, it is reduced during development to the coccyx, or tailbone. Similar structures during development support common ancestry.
